Conventionally, electric motors are known in which permanent magnets are accommodated in magnet accommodating apertures of a rotor core, slits are disposed at two circumferential end portions of a peripheral core that exists radially outside the magnet accommodating apertures, and a certain area in the center of the magnetic pole portion of the peripheral core is formed only of a magnetic part that has no slits in order to reduce vibration and noise in the electric motor (see Patent Literature 1).